1. Field of the Invention
This invention relates to a free-cutting steel and a fuel injection system component using the same.
2. Description of the Related Art
Pb-containing, free-cutting steel has widely been used in fields where an excellent free-cuttability is required. This sort of highly toxic machinability-improving element such as Pb is, however, becoming more strictly regulated from the recent environmental viewpoint, and efforts for searching substitute steel are becoming more active. Bi has been known as one hopeful machinability-improving element substitutable for Pb. Another known substitutive material is exemplified by a material using S as a major component of the machinability-improving element. The material is aimed at producing an inclusion which mainly comprises an MnS-base compound, and thereby raising machinability and grindability through enhancing stress concentration effect on the inclusion in the process of forming cutting chips and lubricating action between a machine tool and the chips.
On the other hand, there is a strong need for high-fatigue-strength steel in recent trends in weight reduction, downsizing, and performance upgrading of mechanical structures and various components such as vehicle components. Under fatigue stress exerted on steel in a fatigue environment, any defects such as inclusion in the texture will cause concentration of the fatigue stress, and fatigue failure of the internal-failure-type will occur as being initiated from the defects. It is therefore necessary to control size and content of the inclusion in the texture, as described in Japanese Laid-Open Patent Publication No. 2003-64412.
The free-cutting steel in which use of the inclusion is welcomed in view of imparting a desirable machinability, and the high-fatigue-strength steel in which the inclusion is not welcomed in view of imparting a high fatigue strength are contradictory with each other in concept of the inclusion. It has, therefore, been extremely difficult to realize a free-cutting steel having a high fatigue strength.
One specific field of application is exemplified by a fuel injection system component. For the fuel injection system, there is a growing demand on increase in the fuel injection pressure in order to meet the emission control law which is becoming more stringent year by year. Any components used in this sort of system will therefore be applied with a larger repetitive stress, so that it is necessary for the component to have a high fatigue strength, and at the same time, a desirable machinability in view of reducing the process costs.
It is therefore an object of the present invention to provide a free-cutting steel capable of suppressing production of coarse inclusions, and having a high fatigue strength and a desirable machinability, and also to provide a fuel injection system component using the same.